I have a 10 gallon tank that had 2 bettas in it with a divider. One of the bettas (Finn) got dropsy. I moved his tank mate (Angel) to a 2.5 gallon as soon as I saw symptoms so that she wouldn't get it. I heard it's highly contagious?
Finn passed away yesterday and I'm hesitant to just drop Angel into the tank they were in. How should I clean it? Will a 50% water change be enough or do I need to start the tank from scratch? I'm not sure of what to do..

I am by no means an expert.. but When I cleaned my tank after a bad bout of ick that killed all of my fish years ago, I boiled the gravel in an old pot for about 5-10 minutes, soaked the plants in a diluted bleach mixture and washed out the tank the old fashioned way with soap and a scrubber then used the same bleach method (I forget the ratio, but I think it's like 19/1 or something?) and then washed it out again with water and let air dry and Never had a problem since.

Keep in mind that doing this will most certainly kill your cycle and you'll have to redo it.